The best way to keep yourself from squeezing while practicing and getting a feel is to push your thumb nail gently against the back wall of the thumb hole. Assuming the ball fit is good, this will keep the ball secure to your hand during the swing, but will keep you from being able to bend your thumb to grip it when its not needed. If the ball is falling off really early you might need to add some tape.
That's really great advice regarding the thumb nail gently pressing against the back of the thumb hole. For years I had 1/4" reverse pitch in the thumb for all my balls. After doing a lot of reading on pitches on this site and others, I made the move to 1/8" forward pitch on my new ball (against the
Pro Shop guy's advice). The transition was a bit rough at first. Thumb was hanging in the ball every so often. However, the thumb nail to the back of the thumb hole advice did wonders to clean up my release and help me not hang in the ball. Thanks, gentlemen.